Metal-organic framework materials based on icosahedral boranes and carboranes

ABSTRACT

Disclosed herein are metal-organic frameworks of metals and boron rich ligands, such as carboranes and icosahedral boranes. Methods of synthesizing and using these materials in gas uptake are disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application No.60/962,195, filed Jul. 27, 2007, which is incorporated herein byreference in its entirety.

STATEMENT OF GOVERNMENTAL SUPPORT

This invention was made with government support under US Dept. of EnergyGrant No. DE-FG02-01ER15244; and Army Research Office Grant No.W911NF-06-0116. The government has certain rights in this invention.

BACKGROUND

Tailorable inorganic coordination polymers, in particular, metal-organicframeworks (MOFs) comprise an important emerging class of materials. SeeOh, et al Nature 438:651-654 (2005); Min, et al. J. Am. Chem. Soc.122:6834-6840 (2000); Cho, et al. Chem. Commun. 2563-2565 (2006); Wu, etal. J. Am. Chem. Soc. 127:8940-8941 (2005); Gomez-Lor, et al. Chem.Mater. 17:2568-2573 (2005); Kitaura, et al. Angew. Chem., Int. Ed.43:2684-2687 (2004); Evans, et al. Chem. Mater. 13:2705-2712 (2001);Lee, et al. J. Am. Chem. Soc. 127:6374-6381 (2005); Dinca, et al. J. Am.Chem. Soc. 127:9376-9377 (2005); Zhao, et al. J. Am. Chem. Soc.126:15394-15395 (2005); Liu, et al. Angew. Chem. Int. Ed. 46:3278-3283(2007); Chen, et al. Angew. Chem. Int. Ed. 44:4745-4749 (2005); Dinca,et al. J. Am. Chem. Soc. 128:16876-16883 (2006); Choi, et al. J. Am.Chem. Soc. 126:15844-15851 (2004); Lee, et al. Angew. Chem., Int. Ed.43:2798-2801 (2004); Bradshaw, et al. Acc. Chem. Res. 38:273-282 (2005);Kitagawa, et al. Angew. Chem., Int. Ed. 43:2334-2375 (2004); Latroche,et al. Angew. Chem., Int. Ed. 45:8227-8231 (2006); Welch, et al. Angew.Chem., Int. Ed. 46:3494-3496 (2007); and Mulfort, et al. J. Am. Chem.Soc. 129:9604-9605 (2007). MOFs are noteworthy for their structural andchemical diversity, high internal surface areas and often permanentmicroporosity. As such, MOFs have attracted great interest for numerousapplications including ion exchange, heterogeneous catalysis,optoelectronics, gas separation, gas sensing, and gas storage, inparticular H₂ storage. Among the factors useful for attaining highgravimetric uptake of H₂ are small pores, open metal coordination sites,and low framework mass. To date, no metallic, carborane-based frameworkshave been made. Organic, metal-free, high porosity covalent organicframeworks have been reported. See El-Kaderi, et al. Science,316:268-272 (2007). Thus, a need exists for metal organic frameworks,which can be used in gas storage and other applications.

SUMMARY

The present invention provides MOFs, which are polymeric crystallinestructures of metals and boron rich ligands, such as carborane (CB)ligands or icosahedral boranes. These MOFs can have gas uptakeproperties. The CB ligand can be one as depicted in Scheme 1, 2, or 3,below. The metal can be any metal capable of coordinating to a boronrich ligand. The MOF can have solvent molecules coordinated to the metalcenters or can be substantially free of solvent. In one embodiment, MOFsare polymeric crystalline structures of Zn₃(OH)(p-CDC)_(2.5)L_(m),wherein p-CDC is the CB ligand1,12-dihydroxycarbonyl-1,12-dicarba-closo-dodecaborane, L is a solvent,and m is an integer from 0 to 4. In some embodiments, L isdiethylformamide (DEF) or dimethylformamide (DMF). In some cases, m is 2or 4. In various embodiments, the pore size of the MOFs disclosed hereinare about 4 Å to about 11 Å, and in specific embodiments, are about 4.5Å to about 9.5 Å. In various embodiments, the gas uptake of the MOFsdisclosed herein is about 0.5 wt % to about 2.4 wt %, or about 1.3 wt %to about 2.1 wt %, at 77 K and 1 atm.

In another aspect, disclosed herein are compositions of the MOFs andfurther comprising a binder, organic viscosity-enhancing compound,liquid, or combination thereof. In some embodiments, the binder issilica, an oxide of magnesium or beryllium, a clay, or mixtures thereof.In various embodiments, the organic viscosity-enhancing compound iscellulose, starch, polyacrylate, polymethacrylate, polyvinyl alcohol,polyvinylpyrrolidone, polyisobutene, polytetrahydrofuran, or mixturesthereof. In some cases, the liquid is water, methanol, ethanol,propanol, n-butanol, isobutanol, tert-butanol, or mixtures thereof.

In yet another aspect, disclosed herein are methods of storing a gasusing the MOFs or compositions, comprising exposing the MOF orcomposition to a gas of interest under conditions sufficient for the MOFto uptake the gas. Typically, the conditions sufficient includetemperature (less than about 150 K, and preferably less than 100 K) andpressure (about 0.1 atm to about 3 atm, and preferably about 0.5 atm toabout 1.5 atm). In some specific cases, the gas is hydrogen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a crystallographically derived (A) structure of 1, the CBligand p-CDC, a carborane building block of the MOFs disclosed herein;(B) topology and connectivity of 2, a MOF of 1; (C) three dimensionaltopology and connectivity of 2; and (D) space-filling packing diagramsof 2 down b axes with DEF removed from Zn₃OH clusters, and coordinatedDEF molecules are omitted from (B)-(D) for clarity.

FIG. 2A shows PXRD patterns for (bottom) a simulated single-crystalstructure of 2, (middle) 2 as synthesized, and (top) 4, as synthesizedfrom 2 (heated to 300° C.).

FIG. 2B shows the heats of adsorption (ΔH_(ads)) for H₂ in 4 (right) and3 (left), where 3 is derived from 2 (heated to 100° C.).

FIG. 3 shows the adsorption (solid squares) and desorption (emptysquares) isotherms for H₂ uptake by (bottom) chloroform treated 2;(middle) 3; and (top) 4, all at 77 K.

FIG. 4 shows TGA analysis of 2, 4, and 4 resolvated.

FIG. 5 shows Fourier-transform infrared spectra (FTIR) of 1(bottom—first spectrum), 2 (second spectrum), 3 (third spectrum), and 4(top—fourth spectrum).

FIG. 6 shows adsorption (empty squares) and desorption (filled squares)of 4 (top) and adsorption (filled squares) and desorption (emptysquares) of 3 (bottom) of N₂ at 77 K.

FIG. 7 shows pore size distribution of 3 and 4.

FIG. 8 shows the H₂ isotherms of 3 at 87 K (bottom—first line) and 77 K(second line) and of 4 at 87 K (third line) and 77 K (top—fourth line).

FIG. 9 shows the H₂ isotherm of 3 at 77 K and 87 K (filled squares) andthe virial equation fit of the same.

FIG. 10 shows the H₂ isotherms of 4 at 77 K and 87 K (filled squares)and the virial equation fit of the same.

FIG. 11A shows a crystal structure of a MOF of Co₄(OH)₂(p-CDC)₃(DMF)₂.

FIG. 11B shows a crystal structure of a MOF ofZn₂(p-BCPD)₂(ethanol)(H₂O).

FIG. 11C shows a crystal structure of a MOF ofZn₃(OH)(p-bis-CDC)_(2.5)(DMF)₄.

FIG. 12A shows a crystal structure of a MOF of MnCDC(DMF) and FIG. 12Bis the crystal structure with the solvents coordinated.

DETAILED DESCRIPTION

Disclosed herein are MOFs, which are polymeric crystalline structures ofboron rich ligands, such as carboranes (CB) or icosahedral boranes,methods of synthesizing these MOFs, and methods of using these MOFs ingas storage. The MOFs disclosed herein are stable to air and water.Without being bound by theory, it is postulated that this stability isdue to the rigidity of the boron rich ligands and the inability for π-πstacking, as compared to previously reported MOFs.

As used herein, the term “polymeric crystalline structures” refers topolymers of monomers which are metals coordinated to boron rich ligands.The materials produced herein are “crystalline” which refers to theordered definite crystalline structure of the material which is uniqueand thus identifiable by a characteristic X-ray diffraction pattern.

Carboranes are icosahedral carbon-containing boron clusters possessingseveral material-favorable properties including rigidity, thermalstability, and chemical stability. Dicarbon carboranes of the formC_(x)B_(x-2)H_(x) (6≦x≦12) may be regarded as three-dimensionaldelocalized aromatic systems in which surface bonding and core bondingcorrespond to σ-bonding and π-bonding, respectively. See, Hawthorne,Advances in Boron Chemistry, Special Publication No. 201, Royal Societyof Chemistry, London, 82:261 (1997). These compounds can be prepared onthe kilogram scale and have been used for a variety of applications,including boron neutron capture therapy, molecular delivery devices inbiomedicine and molecular motors.

The MOFs disclosed herein are based upon metal coordination of thedeprotonated form of boron rich ligands. Metals that can coordinate toboron rich ligands include transition and lanthanide metals. Specificexamples of metals contemplated include, but are not limited to, anyoxidation state of magnesium, calcium, strontium, barium, radium,aluminum, gallium, indium, thallium, silicon, germanium, tin, lead,arsenic, antimony, scandium, titanium, vanadium, chromium, manganese,iron, cobalt, nickel, copper, zinc, yttrium, zirconium, niobium,molybdenum, technetium, rubidium, rhodium, palladium, silver, cadmium,hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum, gold,mercury, lanthanum, cerium, praseodymium, neodymium, promethium,samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium,thulium, and ytterbium. Specific metals and oxidation statescontemplated for use in the MOFs disclosed herein include, but are notlimited to, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Sc³⁺, Y³⁺, Ti⁴⁺, Zr⁴⁺, Hf⁴⁺, V⁴⁺,V³⁺, Nb³⁺, Ta³⁺, Cr³⁺, Mo³⁺, W³⁺, Mn³⁺, Mn²⁺, Re³⁺, Re²⁺, Fe³⁺, Fe²⁺,Ru³⁺, Ru²⁺, Os³⁺, Os²⁺, Co³⁺, Rh²⁺, Rh⁺, Ir²⁺, Ir⁺, Ni²⁺, Ni⁺, Pd²⁺,Pd⁺, Pt²⁺, Pt⁺, Cu²⁺, Cu⁺, Ag⁺, Au⁺, Zn²⁺, Cd²⁺, Hg²⁺, Al³⁺, Ga³⁺, In³⁺,Tl³⁺, Si⁴⁺, Ge⁴⁺, Ge²⁺, Sn⁴⁺, Sn²⁺, Pb⁴⁺, Pb²⁺, As⁵⁺, As³⁺, As⁺, Sb⁵⁺,Sb³⁺, Sb⁺, Bi⁵⁺, Bi³⁺, and Bi⁺.

Dicarborane (CB) ligands include1,12-dihydroxycarbonyl-1,12-dicarba-closo-dodecaborane (p-CDCH₂ (1);FIG. 1A) and carborane ligands as seen in Schemes 1, 2, and 3, below.Some of the ligands have been designated as B3CB, MCB2, BCPD, p-CDC, andp-bis-CDC. Some of the ligands of Scheme 3 comprise one or more CHgroups. These CH groups can be transformed to be C(COOH) groups,allowing for coordination of metal ions via the carboxylate moiety.

p-CDC²⁻ has approximately the same 2-D footprint asbenzene-1,4-dicarboxylate (bdc), the strut defining the archetypal cubicframework compound, MOF-5 [Zn₄O(bdc)₃]_(n). In comparison with otherframeworks, MOF-5 is a good, albeit not spectacular, hydrogen storagematerial, at least at cryogenic temperatures (1.25 wt % at 77K and 1atm). See Panella, et al. Adv. Mater. 17:538-541 (2005). By replacingbenzene with the three-dimensional carborane (volume approximately thatswept out by a rotating phenyl ring), a close structural analogue ofMOF-5 can be obtained, but with smaller pores and therefore higher heatsof adsorption. Such a change can lead to higher hydrogen loading at lowtemperatures and modest pressures and/or more persistent loading athigher temperatures. See Pan, et al. J. Am. Chem. Soc. 128:4180-4181(2006).

The CB ligands can then be coordinated with a metal to form MOFs. TheMOFs can have other ligands incorporated into the polymeric structure,such as hydroxide (OH⁻) anions, or coordinated solvents. The number ofcoordinated solvents will depend upon the method of synthesizing theMOF. The longer the MOF is heated or the higher the temperature to whichit is heated will result in fewer solvent molecules than for MOFs thatwere not heated or were heated at lower temperatures or for shorterperiods of time. The solvent that is coordinated can be any solventwhich is compatible with the synthesis of the monomeric metallic CBintermediates or during formation of the polymeric MOF itself.Non-limiting examples of such solvents include L can be any solventcompatible with the MOF material, or used during its synthesis. Somenon-limiting examples of suitable solvents include diethylformamide(DEF), dimethylformamide (DMF), N-methylpyrrolidone (NMP),tetrahydrofuran, and pyridine.

The MOFs disclosed herein can be one dimensional (1D), two dimensional(2D), or three dimensional (3D). 1D MOFs are linearly aligned repeatingunits of the metal-ligand. 2D MOFs are repeating units of themetal-ligand arranged into sheet-like morphologies. The 3D MOFs aremetal-ligand units arranged in a manner that is neither sheet-like norlinear. Regardless of the arrangement, the MOFs can then be arrangedfurther into stacks or layers.

In some cases, the MOFs disclosed herein are substantially free ofsolvents. As used herein “substantially free” means that solvents arepresent in the MOF at levels less than 1 wt % by weight of the MOF, andpreferably from 0 wt % to about 0.5 wt % by weight of the MOF. Thesolvent can be removed from the MOF by exposing the MOF to elevatedtemperatures under reduced pressure, or by soaking the MOF in a lowboiling solvent to exchange the coordinated solvent for the low boilingsolvent, then exposing the MOF to reduced pressure. The amount ofsolvent in the MOF can be determined by elemental analysis or otherknown analytical techniques.

One class of MOFs disclosed are polymeric structures of repeating unitsof Zn₃(OH)(p-CDC)_(2.5)L_(m), where L is a solvent molecule, as definedabove, and m is an integer from 0 to 4. In preferred embodiments, L isDEF or DMF. M can be 0, 1, 2, 3, or 4.

The pore size of the MOFs as disclosed herein can be altered dependingupon the number of solvent molecules coordinated or partiallycoordinated to the metal center. Typically, the pore size of the MOFwill be about 4.5 Å to about 11 Å, but can be about 4.5 Å to about 9.5Å. In preferred embodiments, the most prevalent pore size of the MOF isabout 4.5 Å to about 5.5 Å. In some preferred embodiments, more than50%, more than 60%, more than 70%, more than 80%, or more than 90% ofthe pore sizes of the MOF are 10 Å or less.

The Brunauer, Emmett, and Teller (BET) surface area of the MOFsdisclosed herein can be about 100 to about 4000 m²/g. In some cases, theBET surface area about 100 to about 2500 m²/g, about 150 to about 2000m²/g, about 150 to about 1500 m²/g, about 150 to about 1000 m²/g, andabout 100 to about 250 m²/g.

Initial synthesis of MOFs as disclosed herein typically result in anumber of fully or partially coordinated solvent molecules, e.g., a MOFdesignated 2. Upon exposure to elevated temperatures (e.g., 100° C. orhigher) and decreased pressures, at least a portion of the solventmolecules are de-coordinated, providing MOF structures that haveexcellent adsorption properties. The removal of at least some of thecoordinated solvent ligands provides for increase gas uptake. Forexample, upon exposure to 100° C. under vacuum, a MOF as disclosedherein, designated 3, has a gas uptake of about 0.7 wt % at 77K and 1atm, whereas the same material, when exposed to 300° C. under vacuum,provides a MOF designated 4 that exhibits a gas uptake of about 2.1 wt %at 77 K and 1 atm. The MOFs disclosed herein typically have a gas uptakeof about 0.5 wt % to about 2.4 wt % at 77K and 1 atm. In some specificexamples, the gas uptake of the MOF is about 0.7 wt % to about 2.2 wt %,about 1.0 wt % to about 2.1 wt %, or about 1.3 wt % to about 2.1 wt %.

Crystal structures of some contemplated MOFs are seen in FIG. 11. FIG.11A shows the crystal structure of a MOF of Co and

Another aspect are compositions comprising the MOFs as disclosed herein.The compositions can include one or more MOF as disclosed herein and abinder, an organic viscosity-enhancing compound, and/or a liquid forconverting the MOF into a paste. The composition can then be used as ameans of storing gas, by exposing the composition to a gas and allowingthe MOF of the composition to uptake the gas.

A number of inorganic compounds can be used as binders. Non-limitingexamples include titanium dioxide, hydrated titanium dioxide, hydratedalumina or other aluminum-containing binders, mixtures of silicon andaluminum compounds, silicon compounds, clay minerals, alkoxysilanes, andamphiphilic substances. Other binders are in principle all compoundsused to date for the purpose of achieving adhesion in powdery materials.Compounds, in particular oxides, of silicon, of aluminum, of boron, ofphosphorus, of zirconium and/or of titanium are preferably used. Ofparticular interest as a binder is silica, where the SiO₂ may beintroduced into the shaping step as a silica sol or in the form oftetraalkoxysilanes. Oxides of magnesium and of beryllium and clays, forexample montmorillonites, kaolins, bentonites, halloysites, dickites,nacrites and anauxites, may furthermore be used as binders. Specificexamples include tetramethoxysilane, tetraethoxysilane,tetrapropoxysilane and tetrabutoxysilane, the analogoustetraalkoxytitanium and tetraalkoxyzirconium compounds and trimethoxy-,triethoxy-, tripropoxy- and tributoxy-aluminum. The binder may have aconcentration of from 0.1 to 20% by weight. Alternatively, no binder isused.

In addition, organic viscosity-enhancing substances and/or hydrophilicpolymers, e.g. cellulose or polyacrylates can be used. The organicviscosity-enhancing substance used may likewise be any substancesuitable for this purpose. Those preferred are organic, in particularhydrophilic polymers, e.g., cellulose, starch, polyacrylates,polymethacrylates, polyvinyl alcohol, polyvinylpyrrolidone,polyisobutene and polytetrahydrofuran.

There are no restrictions with regard to the optional liquid which maybe used to create a paste-like composition of the MOFs disclosed herein.In addition to water, alcohols may be used. Accordingly, bothmonoalcohols of 1 to 4 carbon atoms and water-miscible polyhydricalcohols may be used. In particular, methanol, ethanol, propanol,n-butanol, isobutanol, tert-butanol and mixtures of two or more thereofare used.

Amines or amine-like compounds, for example tetraalkylammonium compoundsor aminoalcohols, and carbonate-containing substances, such as calciumcarbonate, may be used as further additives in the disclosedcompositions. Such further additives are described in EP-A 0 389 041,EP-A 0 200 260 and WO 95/19222, which are incorporated fully byreference in the context of the present application.

Most, if not all, of the additive substances mentioned above may beremoved from the composition by drying or heating, optionally in aprotective atmosphere or under vacuum. In order to keep the MOF intact,the composition is preferably not exposed to temperatures exceeding 300°C. Heating/drying the composition under the mild conditions, inparticular drying in vacuo, preferably well below 300° C. is sufficientto at least remove organic compounds out of the pores of the MOF.Generally, the conditions are adapted and chosen depending upon theadditive substances used.

The order of addition of the components (optional solvent, binder,additives, MOF material) is not critical. It is possible either to addfirst the binder, then, for example, the MOF material and, if required,the additive and finally the mixture containing at least one alcoholand/or water or to interchange the order with respect to any of theaforementioned components.

Additional aspects and details of the disclosure will be apparent fromthe following examples, which are intended to be illustrative ratherthan limiting.

EXAMPLES

Starting materials were purchased from Sigma-Aldrich (ACS grade) andused without further purification unless otherwise noted. Diethyl ether(Et₂O) was purified by published methods (Armarego, et al., Purificationof Laboratory Chemicals, Butterworth-Heinemann: Oxford, 1996; andPangborn, et al., Organometallics, 15:1518 (1996)) and deoxygenated withnitrogen prior to use. Deuterated solvents were purchased and used asreceived from Cambridge Isotopes Laboratories. p-Carborane was providedby Professor M. F. Hawthorne.

Analytical thin layer chromatography (TLC) was performed using glassplates pre-coated with silica gel (0.25 mm, 60 Å pore size) with afluorescent indicator (254 nm). Visualization was accomplished with UVlight and/or palladium chloride (PdCl₂) in 6 M hydrochloric acid as astain.

Powder X-ray diffraction (PXRD) patterns were recorded with a Rigaku XDS2000 diffractometer using nickel-filtered Cu Kα radiation (λ=1.5418 Å).Thermogravimetric analyses (TGA) were performed on a Mettler-ToldeoTGA/SDTA851e. Absorption isotherms were measured with an Autosorb 1-MPfrom Quantachrome Instruments. Infrared spectra (FTIR) were obtained ona BIO RAD FTS-60 spectrophotometer. Elemental analysis was performed byAtlantic Microlab, INC. (Norcross, Ga.). ¹H NMR and ¹³C NMR were done ona Varian Inova 500 spectrometer at 500 MHz and 125 MHz, respectively.¹¹B NMR was done on a Varian Inova 400 spectrometer at 128.5 MHz. NMRsplitting patterns are designated as singlet (s), doublet (d), triplet(t), quartet (q). Splitting patterns that could not be interpreted oreasily visualized are designated multiplet (m) or broad (br). Couplingconstants are reported in Hertz (Hz).

Example 1

Preparation of 1,12-Dihydroxycarbonyl-1,12-dicarba-closo-dodecaborane(1, p-CDC)

Butyl lithium (1.6 M, 35 mL, 56 mmoles) was added via syringe to 2 grams(13.9 moles) of 1,12-dicarba-closo-dodecacarborane (p-carborane)dissolved in 150 mL of dry diethyl ether at 0° C. The reaction mixturewas warmed to room temperature and refluxed for 1.5 hours. The reactionwas then cooled to −78° C. using a dry ice/acetone bath. Carbon dioxidewas bubbled into the reaction mixture for an hour while stirring. Thereaction mixture was concentrated, and 3 M hydrochloric acid (100 mL)was added to the resulting white solid. The precipitate was filtered andwashed with chilled water, hexanes, then chloroform. The product 1 wasobtained as a white solid (2.81 g, 87% yield) and dried in vacuoovernight. Single crystals of 1 were grown from ethanol:water (1:1) byslow evaporation over several days. ¹H NMR (d₈-THF, 500 MHz): δ 11.69(bs, 2H, COOH), 3.2-1.6 (m, 10H, BH); ¹³C NMR (d₈-THF, 125 MHz): δ 162.5(s, COOH), 162.5 (s, BC); ¹¹B{¹H} NMR (CDCl₃): δ −13.9.

Preparation of [Zn₃(OH)(p-CDC)_(2.5)(DEF)₄]_(n) (2)

A small scale amount of [Zn₃(OH)(p-CDC)_(2.5)(DEF)₄]_(n) (2) wasprepared as follows. Exact amounts of Zn(NO₃).6H₂O (33 mg, 0.11 mmole)and 1,12-dicarboxylic-1,12-dicarba-closo-dodecaborane (8.3 mg, 0.035mmole) were dissolved in 1 mL dimethylformamide (DMF). The solution washeated at 80° C. for 24 hours. Larger scale amounts were prepared asfollows. Exact amounts of Zn(NO₃).6H₂O (1.20 g, 4.03 mmole) and1,12-dicarboxylic-1,12-dicarba-closo-dodecaborane (0.30 g, 1.28 mmole)were dissolved in 36 mL 1:1 DMF:ethanol. The solution was heated at 80°C. for 24 hours. The crystals were collected by filtration, washed withDMF and ethanol, and dried in air, providing 480 mg 2 (32% yield basedon zinc). The crystals of 2 were also heated at 100° C. in vacuo toprovide 3.

Preparation of DEF-Free MOF Based Upon 2 (4)

Crystals of 2 were heated in vacuo at 300° C. for 24 hours to produce 4.Anal. Calcd. for Zn₃B₂₅C₁₀H₃₀O₁₃ (4.H₂O)C, 14.3; H, 3.6; N, 0.0. Found:C, 13.95; H, 3.47; N, 0.0.

Analysis of Properties of 3 and 4

The solvathermal synthesis disclosed herein yielded a complex MOF of theformula [Zn₃(OH)(p-CDC)_(2.5)(DEF)₄]_(n) (2) [DEF=diethylformamide](FIG. 1). X-ray analysis of a single crystal of 2 revealed a structurein which two of the three zinc ions are coordinated to two DEF moleculeseach in an octahedral geometry. In addition, one of the dicarboxylateligands in the structure is ligated to zinc through only one oxygenatom. The zincs are further connected by a triply bridging hydroxideion. Thermogravimetric analysis (TGA) of 2 revealed mass losses between125-175° C. and 175-250° C., assigned to free and coordinated DEFrespectively, but no further mass loss up to 500° C. (See FIG. 4.)Elemental analysis measurements of the crystalline material heated undervacuum at 300° C. confirmed the removal of the coordinated DEF. Powderx-ray diffraction (PXRD) measurements established that although thecrystallinity is retained, the structure is irreversibly altered.Although a single-crystal structure of the DEF-free version of the MOF(4) has yet to be isolated (FIG. 2A), infrared data strongly suggestthat the partially coordinated carboxylate of 2 becomes fullycoordinated in 4 (see FIG. 5). Nonetheless, the number of coordinationsites occupied by DEF in 2 is greater than the number of coordinationsites needed for complete coordination of p-CDC²⁻. This mismatch mayresult in coordinatively unsaturated or at least highly reactive metalsites.

Adsorption measurements (FIG. 6) were used to determine theN₂-accessible surface areas of 3 (compound 2 evacuated at 100° C.) and4. The Brunauer, Emmett, and Teller (BET) surface areas are 248 and 152m²/g, respectively. Notably, despite the removal of coordinated DEF, theconversion of 3 to 4 decreases the size of the most prevalent pores from6 to 5 Å (FIG. 7).

In contrast to the modest N₂ adsorption, 4 has high H₂ uptake at 77K:2.1% at 1 atm. This uptake is triple the uptake of 3 (volumetricmeasurements; FIG. 3) and stands in striking contrast to unheatedsamples of 2, which showed no uptake even after one week of exposure tochloroform, a solvent which is often effective for solvent exchange, andambient vacuum. Further comparison of 3 and 4 revealed that at 1 atm thelatter takes up about 6 additional H₂ molecules per [Zn₃(OH)]₇— cluster.Without intending to be bound by theory, the enhanced uptake may beattributable to putative open metal sites, and/or the reduction of poresize may also play a role. The H₂ results for 4 compare favorably tothose for a variety of other MOFs measured under the same conditions andindeed are exceeded by only three other framework materials (See Liu, etal. Angew. Chem. Int. Ed. 46:3278-3283 (2007); Chen, et al. Angew. Chem.Int. Ed. 44:4745-4749 (2005); and Dinca, et al. J. Am. Chem. Soc.128:16876-16883 (2006)), where the highest uptake reported was 2.45%.

Isosteric heats of adsorption for H₂ in 3 and 4 were obtained by fitting77 and 87K isotherms to appropriate virial equations (FIG. 2B). SeeCzepirski, et al. Chem. Eng. Sci. 44:797-801 (1989). ΔH_(ads)(H₂) valuesfor 4 are substantially higher than for 3 over the entire loading range.These results are likewise qualitatively consistent with effectsexpected from reduction of pore size and/or formation of open metalcoordination sites.

The first carborane-based MOF (2) has been synthesized. Removal of thecoordinated solvent molecules triples the uptake of H₂ by the materialat 77K and 1 atm (4 vs. 3) despite a decrease in surface area andreduction of N₂-accessible pore volume. The resistance to pore collapseupon conversion of 3 to 4 likely reflects the rigidity of thedicarborane, its 3-dimensional sterics, and its inability to benefitgreatly from stacking-type (collapsed structure) van der Waalsinteractions.

Single crystals were mounted on a Bruker SMART CCD 1000 diffractometerequipped with a graphite-monochromated MoKα (λ=0.71073 Å) radiationsource in a cold nitrogen stream. All crystallographic data werecorrected for Lorentz and polarization effects (SAINT), and face-indexabsorption. The structures were solved by direct methods and refined bythe full-matrix least squares method on F² with appropriate softwareimplemented in the SHELXTL program package. In the structure of 2, threeof the four coordinated DEF molecules could be reasonably modeled.Remaining contributions from disordered DEF coordinated to the node andwithin the pores were removed by the SQUEEZE routine (PLATON). All thenon-hydrogen atoms were refined anisotropically. Hydrogen atoms on thecarborane cage in 2 and 1 were found in the difference map and hydrogenatoms on the solvent molecules were added at their geometrically idealpositions. One ethanol molecule is reasonably modeled in 1.

Crystal Data and Structure Refinement for 1 and 2 Compound 1 2 Empiricalformula C₄H₁₂B₁₀O₄ C₂₅H₅₉B₂₅N₃O₁₅Zn₃ Formula weight 232.24 1108.11Crystal color, habit Colorless, plate Colorless, plate Crystal dimension(mm³) 0.170 × 0.123 × 0.045 0.350 × 0.284 × 0.063 Crystal systemtriclinic orthorhombic Space group P-1 Pbcn a (Å)  6.6388(10) 39.356(5)b (Å)  7.1089(11) 14.5154(18) c (Å)  7.1799(11) 21.629(3) α (deg)78.937(2) 90 β (deg) 74.844(2) 90 γ (deg) 62.838(2) 90 V (Å³) 289.96(8) 12356(3) Z 1 8 ρ (calcd, g/cm³) 1.330 1.191 μ (cm⁻¹) 0.086 1.205Goodness-of-fit on F² 1.079 1.048 R 0.0523 0.0655 R_(W) 0.1398 0.1807Absorption Measurements

Samples of known weight were evacuated under 10⁻⁵ torr dynamic vacuumfor 24 hours on an Autosorb 1-MP from Quantachrome Instruments prior togas absorption measurements. The evacuated sample was weighed again toobtain the sample weight.

Isosteric Heat of Absorption

The hydrogen isotherms obtained at 77 K and 87 K (FIG. 8) were fit tothe following virial equation (Czepirski, et al., J Chem Eng Sci, 44:787(1989)):

${\ln\; p} = {{\ln\; N} + {\frac{1}{T}{\sum\limits_{i = 0}^{m}\;{a_{i\;}N^{i}}}} + {\sum\limits_{i = 0}^{n}\;{b_{i}N^{i}}}}$and the resulting heat of absorption is shown in FIG. 9 (for 3) and FIG.10 (for 4).

The heats of adsorption of 3 and 4 were calculated from the fittingparameters in the following equation:

${q_{st}(N)} = {{- R}{\sum\limits_{i = 0}^{m}\;{a_{i}N^{i}}}}$

The FTIR spectra of 1, 2, 3, and 4 are shown in FIG. 5. The peaks forν(CO) at about 1410 and 1640 cm⁻¹ transition to a sharper set of peakswhen the free and coordinated solvent molecules are removed (seespectrum for 4). Elemental analysis indicates the complete removal ofDEF (or DMF) solvent molecules. In structure 4 ν(CO) peaks becomesignificantly sharper compared to structure 3, which indicates completecoordination of the ligands of 4. The peaks in 4 are consistent with thepresence of mainly fully coordinated carboxylates. There is also a smallpeak near 1700 cm⁻¹ that may correspond to a stretch for anon-coordinated C═O (see spectrum for 1). The crystal structure of 2shows a ratio of one partially coordinated COO⁻ unit to five fullycoordinated COO⁻ units.

Example 2 Preparation of Other Carborane Ligands

Other carborane ligands for use in MOFs as disclosed herein can beprepared from p-carborane, as outlined in the following scheme.

Synthesis of the carborane ligand precursors as seen in Scheme 1,p-bis-CDC 6, p-CDC 1, and BSPD 4, were as follows. For step (i)conditions: mixture of carborane and butyl lithium in ether and exposureto CO₂, cooled from 0° C. to −80° C. The yield of p-CDC was 80%. Forstep (ii) p-carborane, 1,4-iodobenzene, and butyl lithium were reactedin ether and warmed from 0° C. to room temperature. Then copper (I)chloride in pyridine was added and the mixture refluxed to provide theintermediate shown, in a yield of 40%. The conditions for step (iii)were, mixing p-carborane, and butyl lithium in ether with copper(I)chloride at reflux to provide the intermediate in 72% yield. For step(iv), the intermediate from step (ii) was reacted with butyl lithium inether at 0° C. and exposed to CO₂ and cooled to −80° C., to provide BSPDin 95% yield. For step (v), the intermediate of step (iii) was mixedwith methyl lithium in ether and exposed to CO₂ and cooled from 0° C. to−80° C. to provide p-bis-CDC in 95% yield.

The CB ligand of Scheme 5, B3CB, is prepared as follows. The carboraneis reacted with iodine and aluminum chloride in methylene chloride toprovide the intermediate after step (i). Then, a coupling reaction isperformed with the intermediate, magnesium, 4-iodo toluene, in thepresence of PdCl₂(PPh₃)₂ at reflux in ether. For step (iii), theintermediate from step (ii) is reacted with butyl lithium in ether withcopper chloride at reflux to provide the tris(toluene) intermediate.Next, for step (iv), the tris(toluene) intermediate is reacted withsulfuric acid and chromium oxide in acetone to provide the CB ligandB3CB.

The synthesis of the metallo-(bis)dicarbollide CB (MCB2) ligand shownabove proceeds as follows. M stands for any metal capable ofcoordinating to the intermediate after step (iv), e.g., nickel, copper,cobalt, iron, or zinc. The conditions for step (i) are exposing thecarborane to iodine and aluminum chloride in methylene chloride to formthe iodide intermediate. Next, for step (ii) the intermediate is reactedwith methyl magnesium bromide and Pd(PPh₃)₂Cl₂ in ether at reflux. Theethyl intermediate is then treated with tetrabutylammonium fluoride(TBAF) at reflux in THF to provide a singly deprotonated intermediatewhich is then reacted immediately with sodium hydride to provide thedoubly deprotonated intermediate. Coordination with a metal ion, e.g.,Ni, Co, Cu, Fe, Zn, by mixing with the appropriate metal salt providesthe intermediate after step (v). Lastly, the ethyl group is oxidized toa carboxylate by treatment with CrO₃ and H₂SO₄ in acetone.

The MOFs based upon these CB ligands are prepared as follows. The ligandis mixed with the corresponding metal salt (a transition metal orlanthanide) in a molar proportion of 1:n, where n is greater than orequal to 1 in an organic solvent or mixture of organic solvents, such asdimethylformamide, diethylformamide, ethanol, isopropanol, methanol,butanol, or pyridine. The mixture is reacted until crystalline materialis formed. Then, the solvent is decanted and the resulting CB-MOF iscollected and washed several times with organic solvent to afford theCB-MOF material. The CB-MOF can then be further modified by removing thecoordinated solvent molecules under elevated temperature and reducedpressure. Confirmation of removal of all solvent molecules from theCB-MOF can be confirmed via elemental analysis.

For gas sorption applications, the CB-MOF is evacuated at 25-400° C.,under vacuum for about 1 to 48 hours to afford activated correspondingCB-MOF materials. Alternatively, activation can proceed via soaking theCB-MOF in a low boiling solvent, such as chloroform, dichloromethane, oracetone for about 1 to 48 hours. This soaking is followed by vacuumevacuation of the low boiling solvent. In activated form, the CB-MOFexhibits a high surface area (e.g., about 150-1500 m²/g and up to about4000 m²/g).

1. A metal-organic framework (MOF) comprising a polymeric crystallinestructure of Zn₃(OH)(p-CDC)_(2.5)L_(m), wherein p-CDC is1,12-dihydroxycarbonyl-1,12-dicarba-closo-dodecaborane, L is a solvent,and m is an integer from 0 to
 4. 2. The MOF of claim 1, wherein m isdifferent from 0 and at least one L is diethylformamide.
 3. The MOF ofclaim 2, wherein m is different from 0 or 1 and each L isdiethylformamide.
 4. The MOF of claim 1, wherein m is
 2. 5. The MOF ofclaim 1, wherein m is
 4. 6. The MOF of claim 1 having a pore size ofabout 4 to about 11 Å.
 7. The MOF of claim 6, wherein the pore size isabout 4.5 to about 9.5 Å.
 8. The MOF of claim 1 having a hydrogen gasuptake of about 0.5 wt % to about 2.4 wt % at 77 K and 1 atm.
 9. The MOFof claim 8, wherein the H₂ uptake is about 1.3 wt % to about 2.1 wt %.10. A composition comprising the MOF of claim 1 and one or more of abinder, an organic viscosity-enhancing compound, and a liquid.
 11. Thecomposition of claim 10, wherein the binder is selected from the groupconsisting of silica, an oxide of magnesium, an oxide of beryllium, aclay, and mixtures thereof.
 12. The composition of claim 10, wherein theorganic viscosity-enhancing compound is selected from the groupconsisting of cellulose, starch, polyacrylate, polymethacrylate,polyvinyl alcohol, polyvinylpyrrolidone, polyisobutene,polytetrahydrofuran, and mixtures thereof.
 13. The composition of claim10, wherein the liquid is selected from the group consisting of water,methanol, ethanol, propanol, n-butanol, isobutanol, tert-butanol, andmixtures thereof.
 14. A method of storing a gas comprising exposing agas to a MOF of claim 1 under conditions sufficient for the MOF touptake the gas.
 15. The method of claim 14, wherein the MOF uptakes thegas at about 0.5 wt % to about 2.5 wt % at 77 K and 1 atm.
 16. Themethod of claim 14, wherein the gas is hydrogen.
 17. A metal-organicframework (MOF) comprising a polymeric crystalline structure of a metaland a carborane ligand or an icosohedral borane ligand, and optionally asolvent.
 18. The MOF of claim 17 substantially free of a solvent. 19.The MOF of claim 17, wherein the metal is selected from the groupconsisting of Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Sc³⁺, Y³⁺, Ti⁴⁺, Zr⁴⁺, Hf⁴⁺, V⁴⁺,V³⁺, Nb³⁺, Ta³⁺, Cr³⁺, Mo³⁺, W³⁺, Mn³⁺, Mn²⁺, Re³⁺, Re²⁺, Fe³⁺, Fe²⁺,Ru³⁺, Ru²⁺, Os³⁺, Os²⁺, Co³⁺, Rh²⁺, Rh⁺, Ir²⁺Ir⁺, Ni²⁺, Ni⁺, Pd²⁺, Pd⁺,Pt²⁺, Pt⁺, Cu²⁺, Cu⁺, Ag⁺, Au⁺, Zn²⁺, Cd²⁺, Hg²⁺, Al³⁺, Ga³⁺, In³⁺,Tl³⁺, Si⁴⁺, Ge⁴⁺, Ge²⁺, Sn⁴⁺, Sn²⁺, Pb⁴⁺, Pb²⁺, As⁵⁺, As³⁺, As⁺, Sb⁵⁺,Sb³⁺, Sb⁺, Bi⁵⁺, Bi³⁺, Bi⁺, and mixtures thereof.
 20. The MOF of claim17, wherein the carborane ligand is selected from the group consistingof

and mixtures thereof, wherein M is selected from the group consisting ofnickel, copper, cobalt, iron, and zinc.
 21. The MOF of claim 17, whereinthe metal comprises zinc, nickel, cobalt, manganese, or a mixturethereof.
 22. The MOF of claim 17, wherein the carborane ligand comprises

and the metal comprises zinc.
 23. The MOF of claim 22 comprisingZn₂(BCPD)₂L_(m), wherein L is a solvent and m is an integer from 0 to 4.24. The MOF of claim 17, wherein the carborane ligand comprises

and the metal comprises zinc.
 25. The MOF of claim 24 comprisingZn₃(OH)(p-bis-CDC)_(2.5)L_(m), wherein L is a solvent and m is aninteger from 0 to
 4. 26. The MOF of claim 17, wherein the carboraneligand comprises

and the metal comprises cobalt.
 27. The MOF of claim 26 comprisingCo₄(OH₂)(p-CDC)₃L_(m), wherein L is a solvent and m is an integer from 0to
 4. 28. The MOF of claim 17, wherein the carborane ligand comprises

and the metal comprises manganese.
 29. The MOF of claim 28 comprisingMn(p-CDC)L_(m), wherein L is a solvent and m is an integer from 0 to 4.30. The MOF of claim 17 having a surface area of about 100 to about 4000m²/g.
 31. The MOF of claim 30 having a surface area of about 150 toabout 1500 m²/g.
 32. A method of storing a gas comprising exposing thegas to the MOF of claim 17 under conditions sufficient for the MOF touptake the gas.